A wide variety of coatings are used to coat both the interior and/or exterior surfaces of packaging articles. The coatings serve a variety of functions that may vary depending on factors such as, for example, the nature of the packaging article, the nature of the good to be packed within the packaging article, and the nature of the substrate onto which the coating is to be applied. Such coatings are frequently applied to the interior of metal food or beverage containers to prevent or retard the interaction of food or beverage products with metal substrates of the containers.
A variety of manufacturing processes are used to apply such coatings to substrates. For example, portions of metal cans are sometimes coated using “coil coating” or “sheet coating” operations, in which a planar coil or sheet of a suitable substrate is coated with a suitable composition and hardened. The coated substrate is then formed into a can end or body. Alternatively, liquid-coating compositions may be applied (e.g., by spraying, dipping, rolling, etc.) to the formed article and then hardened.
Coatings for packaging applications should preferably be capable of high-speed application to the substrate and provide the necessary properties when hardened to perform in this demanding end use. For example, in some applications, the coatings should have excellent adhesion to the substrate, resist degradation over extended periods of time, and be safe for food contact (for applications requiring such contact), even when exposed to harsh environments.
The food and beverage packaging industry has increasingly employed water-based coating systems in place of organic solvent-based coating systems. Significant problems have been encountered, however, when utilizing conventional water-based coating systems in food or beverage applications that require preservation or sterilization of food or beverage products (e.g., through elevated temperature and pressure conditions). Such problems may include, for example, corrosion of the coating, absorption of water into the coating, staining or discoloring of the coating, and/or loss of adhesion of the coating with the underlying substrate. Moreover, packaged food or beverage products frequently exhibit aggressive chemical properties that may damage conventional water-based coating systems, especially when combined with the elevated temperatures of preservation or sterilization conditions.
Thus, there is a continuing need for improved water-based coating systems that resist such degradation.